Dear enemy effect
The dear enemy effect is an ethological phenomenon in which two neighboring territorial animals become less aggressive toward one another once territorial borders are well-established.Fisher, J., {1954}. Evolution and bird sociality. In: Evolution As a Process (Huxley, J., Hardy, A. and Ford, E., eds). London, Allen and Unwin. pp. 71-83 As territory owners become accustomed to their neighbors, they expend less time and energy on defensive behaviors directed toward one another. However, aggression toward unfamiliar neighbors remains the same. Some authors have suggested the dear enemy effect is territory residents displaying lower levels of aggression toward familiar neighbors compared to unfamiliar individuals who are non-territorial "floaters".Bee, M.A., (2003). A test of the "dear enemy effect" in the strawberry dart-poison frog (Dendrobates pumilio). Behavioral Ecology and Sociobiology, 54: 601-610 The dear enemy effect has been observed in a wide range of animals including mammals, birds, reptiles, amphibians, fish and invertebrates. The behavioural effect can be modulated by factors such as the location of the familiar and unfamiliar animal, the season, and the presence of females. Function The ultimate function of the dear enemy effect is to increase the individual fitness of the animal expressing the behaviour. This increase in fitness is achieved by reducing the time, energy or risk of injury unnecessarily incurred by defending a territory or its resources (e.g. mate, food, space) against a familiar animal with its own territory; the territory-holder already knows about the abilities of the neighbour, and also knows that the neighbour is unlikely to try to take over the territory because it already has one. Mechanism The interaction between two neighbours can be modelled as a prisoner's dilemma game. In this view, a territory owner that acts non-aggressively towards a neighbour can be thought of as cooperating, while a territory owner that acts aggressively towards its neighbour can be considered to have defected. A necessary condition for the prisoner’s dilemma game to hold is that an aggressive individual should enjoy greater benefits than a non-aggressive individual when each is faced with a non-aggressive opponent. This stipulation is plausible, as an aggressive individual might enlarge their territory or steal food or matings from a non-aggressive individual. When cooperation involves a cost, a possible mechanism for achieving stable co-operation is reciprocal altruism, where pairs of individuals trade bouts of cooperative behaviour with one another. Dear enemy cooperation could be explained by reciprocal altruism if territorial neighbours use conditional strategies such as tit-for-tat. In the tit-for-tat strategy, a subject will cooperate when its partner (neighbour) cooperates and defect when the partner defects.Hyman, J., (2002). Conditional strategies in territorial defense: do Carolina wrens play tit-for-tat? Behavioral Ecology, 13: 664-669. DOI: 10.1093/beheco/13.5.664 Occurrence In mammals Territorial Eurasian beavers (Castor fiber) presented with a two-way choice sniffed both castoreum and anal gland secretion from a stranger longer than from a neighbour. Furthermore, beavers responded aggressively (stood on the mound on their hind feet, pawing and/or overmarking) longer to castoreum, but not to anal gland secretion, from a stranger than from a neighbour. When the mounds containing the scents were allowed to remain overnight and the beavers' responses measured the following morning, the beavers' responses were stronger to both castoreum and anal gland secretion from a stranger than from a neighbour.Rosell, F. and Bjørkøyli, T. (2002). A test of the dear enemy phenomenon in the Eurasian beaver. Animal Behaviour, 63: 1073–1078 Eurasian badgers (Meles meles) can discriminate between self-, neighbour- and unfamiliar- group faeces near their main sett. Badgers show heightened behavioural responses towards unfamiliar- compared with self-group scents, but there is no difference in response to neighbour- relative to self-group scents. The relative responses towards unfamiliar-group scents are greatest during the breeding seasons, but there is no seasonal differences in the responses to neighbour-group versus self-group scents. In badger populations, levels of aggression between neighbouring territory-holders are likely to be kept relatively low through neighbour recognition. However, increased levels of aggression will be shown towards dispersing or itinerant (alien) badgers, especially during periods such as the breeding season when the potential threats to the long-term fitness of territory owners are greatest.Palphramand1, K.L. and White, P.C.L., (2007). Badgers, Meles meles, discriminate between neighbour, alien and self scent. Animal Behaviour, 74: 429–436 Red squirrels are able to discriminate the odours of familiar neighbours and strangers.Vaché, M., Ferron, J. and Gouat, P., (2001). The ability of red squirrels (Tamiasciurus hudsonicus) to discriminate conspecific olfactory signatures. Canadian Journal of Zoology, 79: 1296-1300 It has been suggested that this discrimination may be used by males to avoid unnecessary chases and fights by becoming known to their neighbours. In birds Audio playback studies are often used to test the dear enemy effect in birds. These studies have demonstrated several bird species respond more aggressively to played back songs of strangers than to songs of neighbours including the Alder Flycatcher (Empidonax alnorum),Lovell, S.F. (2004). Neighbor-stranger discrimination by song in a suboscine bird, the alder flycatcher, Empidonax alnorum. Behavioral Ecology, 15: 799-804 DOI: 10.1093/beheco/arh082 male Blue Grouse,Falls, J.B. and McNicholl, M.K., (1979). Neighbor-stranger discrimination by song in male blue grouse. Canadian Journal of Zoology, 57: 457-462 European Robin (Erithacus rubecula),Brindley, E.L., (1991). Response of European robins to playback of song: neighbor recognition and overlapping. Animal Behaviour, 41: 503-512 and male Banded Wren (Thryothorus pleurostictus).Mollesf, L.E. and Vehrencamp, S.L., (2001). Neighbour recognition by resident males in the banded wren, Thryothorus pleurostictus, a tropical songbird with high song type sharing. Animal Behaviour, 61: 119–127 Neighbouring male Song Sparrows (Melodia melospiza) differ individually in their aggressiveness. Increased aggression by residents towards intruders indicates that residents not only respond to intrinsic aggressiveness of their neighbours, but also to short-term changes in aggression levels.Akçay, C. et al., (2009). Good neighbour, bad neighbour: song sparrows retaliate against aggressive rivals. Animal Behaviour, 78: 97–102 Although neighbour–stranger discrimination has been reported in many passerine birds, it has seldom been investigated in territorial non-passerine species. The nocturnal raptor, the Little Owl (Athene noctua), hoots to defend its territory. Male Little Owls respond less to their neighbour's hoots played back from the usual location. However, responses to playback of a neighbour from an unusual location are similar to responses to playback of a stranger's hoots from either location.Hardouin, L.A., Tabel, P. and Bretagnolle, V., (2006). Neighbour–stranger discrimination in the little owl, Athene noctua. Animal Behaviour, 72: 105–112 During the breeding season of the Skylark (Alauda arvensis), particular common sequences of syllables (phrases) are produced by all males established in the same location (neighbours), whereas males of different locations (strangers) share only few syllables. Playback experiments provided evidence for neighbour–stranger discrimination consistent with the dear enemy effect, indicating that shared sequences were recognized and identified as markers of the group identity.Briefer, E., Aubin, T., Lehongre, K. and Rybak, F., (2008). How to identify dear enemies: the group signature in the complex song of the skylark Alauda arvensis. Journal of Experimental Biology, 211: 317-326. DOI: 10.1242/jeb.013359 Studies have shown that the dear enemy effect changes during the breeding season of the skylark. Playbacks of neighbour and stranger songs at three periods of the breeding season show that neighbours are dear enemies in the middle of the season, when territories are stable, but not at the beginning of the breeding season, during settlement and pair formation, nor at the end, when bird density increases due to the presence of young birds becoming independent. Thus, the dear enemy relationship is not a fixed pattern but a flexible one likely to evolve with social and ecological circumstances.Briefer, E., Rybak, F. and Aubin, T., (2008). When to be a dear enemy: flexible acoustic relationships of neighbouring skylarks, Alauda arvensis. Animal Behaviour, 76: 1319–1325 In reptiles ]] Males of a territorial lizard, the tawny dragon (Ctenophorus decresii), reduced their aggression levels in repeat interactions with familiar rivals and increased their aggression levels towards unfamiliar males. The time taken for interactions to be settled was also lower towards familiar than unfamiliar males.Osborne, L., (2005). Rival recognition in the territorial tawny dragon (Ctenophorus decresii). Acta Ethologica, 8: 45-50 Another territorial lizard, the Common collared lizard (Crotaphytus collaris), can individually recognize neighbours and will increase aggression towards them as the threat to territorial ownership increases. Resident males treat familiar neighbours that had been moved to the opposite boundary to the shared boundary as equally aggressive as strangers. However, residents responded more aggressively towards strangers than towards neighbours on natural territories and also in neutral arena encounters.Husakf, J.F. and Fox, S.F., (2003). Adult male collared lizards, Crotaphytus collaris, increase aggression towards displaced neighbours. Animal Behaviour, 65: 391–396 In the brown anole lizard (Anolis sagrei), dyads of males behave differently depending on whether the lizards are prior neighbours, with prior neighbours exhibiting fewer bobbing relative to nodding forms of headbob displays than non-neighbours.McMann, S. and Paterson, A.V., (2012). Display behavior of resident brown anoles (Anolis sagrei) during close encounters with neighbors and nonneighbors. Herpetological Conservation and Biology, 7(1): 27−37 In amphibians Males of the territorial breeding agile frog (Rana dalmatina), have a large variability in call characteristics and are able to discriminate between neighbouring and unfamiliar conspecifics. Calling is of the longest duration in response to an unfamiliar acoustic stimulus; in contrast, the response to a familiar conspecific call does not show any difference from solitary vocalisations.Lesbarrèresa, D. and Lodéa, T., (2002). Variations in male calls and responses to an unfamiliar advertisement call in a territorial breeding anuran, Rana dalmatina: evidence for a “dear enemy” effect. Ethology, Ecology & Evolution, 14: 287-295. DOI:10.1080/08927014.2002.9522731 In fish The dear enemy effect in male variegated pupfish (Cyprinodon variegatus) is dependent on the presence of females. Reduced aggression consistent with dear enemy recognition occurs between conspecific neighbours in the absence of females, but the presence of a female in a male's territory instigates comparably greater aggression between the neighbours.Leiser, J.K., (2003). When are neighbours ‘dear enemies’ and when are they not? The responses of territorial male variegated pupfish, Cyprinodon variegatus, to neighbours, strangers and heterospecifics. Animal Behaviour, 65: 453–462 Some researchers have staged three-way contests between male Convict cichlids (Cichlasoma nigrofasciatum) to examine the dear enemy effect. When faced with a familiar neighbour and an unfamiliar intruder simultaneously, residents preferentially confronted the unfamiliar opponent. That is, the establishment of dear enemy recognition between a resident and a neighbour allowed the resident to direct his aggression to the greater competitive threat, i.e. the intruder.Leiser, J.K. and Itzkowitz, M., (1989). The benefits of dear enemy recognition in three-contender convict cichlid (Cichlasoma nigrofasciatum) contests. Behaviour, 136: 983-1003 Individual recognition of noises produced males of the bicolor damselfish (Pomacentrus partitus) have been demonstrated in the field. Playbacks of non-resident sounds from a given fish's territory elicit a greater response from its nearest neighbour than playbacks of the resident's sound. Testing also included switching the sounds of the two nearest neighbours relative to each respective male's territory. Results demonstrated that all males in the colony individually recognize the sounds of their two nearest neighbours.Myrberg, A.A. and Riggio, R.J., (1985). Acoustically mediated individual recognition by a coral reef fish (Pomacentrus partitus). Animal Behaviour, 33: 411–416 In invertebrates The home ranges of colony living ants often overlap the ranges of other conspecific colonies and colonies of other species. In laboratory experiments, the frequency and severity of agonistic interactions among workers from different colonies increases with the distance between their nests; this has been reported for Leptothorax nylanderi''Hkinzk, J., Foitzik, S., Hippert, A. and Hölldobler, B., (1996). Apparent dear-enemy phenomenon and environment-based recognition cues in the ant Leptothorax nylanderi. Ethology, 102: 510–522. DOI: 10.1111/j.1439-0310.1996.tb01143.x and ''Pheidole ants.Langen, T.A., Tripet, F. and Nonacs, P., (2000). The red and the black: habituation and the dear-enemy phenomenon in two desert Pheidole ants. Behavioral Ecology and Sociobiology, 48: 285-292 The dear enemy effect has been reported in colonies of the fungus-growing termite ''Macrotermes falciger''. Behavioural tests with workers reveal no alarm behaviour or mortality in pairings of workers from the same colony but a full range from no alarm to overt aggression, with associated death, when individuals were paired from different colonies. The level of mortality increases with differences in the composition of cuticular hydrocarbons between colonies.Kaib1, M., Franke, S., Francke, W. and Brand, R., (2002). Cuticular hydrocarbons in a termite: phenotypes and a neighbour–stranger effect. Physiological Entomology, 27, 189–198. DOI: 10.1046/j.1365-3032.2002.00292.x Male sand fiddler crabs (Uca pugilator) defend territories that consist of a breeding burrow and a display area where they wave their claw to attract females. Burrow-holding males engage in agonistic contests with both intruding males that attempt burrow take-overs and with other territory-holding neighbours that apparently attempt to limit waving or other surface activities of rivals. Contests consist of one or more behavioural elements that range from no claw contact to use of the claw to push, grip, or flip an opponent. In the field, contests with intruders begin at higher intensities and escalate more rapidly than those with neighbours. However, resident-resident contests increase in intensity when burrows are close, neighbours faced each other when exiting burrows, and neighbours were of similar size. Proximity and orientation determine the ease with which a neighbour may be engaged.Pratt, A.E. and McLain, D.K., (2006). How dear is my enemy: Intruder-resident and resident-resident encounters in male sand fiddler crabs (Uca pugilator). Behaviour, 143: 597-617 Nasty neighbour effect or no effect A range of studies have found evidence of an effect opposite to the dear enemy effect, i.e. more aggression is shown toward neighbours than strangers. This has been termed the "nasty neighbour" effect. Colonies of the weaver ant (Oecophylla smaragdina) are able to recognize a greater proportion of workers from neighbouring colonies as non-colony members. When recognized as non-colony members, more aggression is exhibited toward neighbours than non-neighbours.Newey, P.S., Robson, S.K. and Crozier, R.H., (2010). Weaver ants Oecophylla smaragdina encounter nasty neighbors rather than dear enemies. Ecology, 91(8):2366-72 banded mongoose (Mungos mungo) groups vocalize more and inspect more scent samples in response to olfactory cues of neighbours than strangers.Müller, C.A. and Manser, M.B., (2007). ‘Nasty neighbours’ rather than ‘dear enemies’ in a social carnivore. Proc. R. Soc. B., 274: 959-965 It has been suggested that increased aggression towards neighbours is more common in social species with intense competition between neighbours, as opposed to reduced aggression towards neighbours typical for most solitary species. Furthermore, animals may respond in this way when encounters with intruders from non-neighboring colonies are rare and of little consequence. Female New Zealand Bellbirds (Anthornis melanura) are more aggressive toward the songs of neighbouring females. This is opposite to the dear enemy phenomenon and suggests that neighbouring females pose a greater threat than strangers in his species.Brunton, D.H., Evans, B., Cope, T. and Ji, W. (2008). A test of the dear enemy hypothesis in female New Zealand bellbirds (Anthornis melanura): female neighbors as threats. Behavioral Ecology, 19 (4): 791-798. DOI: 10.1093/beheco/arn027 http://beheco.oxfordjournals.org/cgi/content/abstract/arn027v1 A range of studies have found no evidence of the dear enemy effect showing the effect is not universal. Territorial males of the strawberry dart-poison frog (Dendrobates pumilio) and the Spotted Antbird (Hylophylax naevioides)Bard, S., Hau, M., Wikelski, M. and Wingfield, J.C. (2002). Vocal distinctiveness and response to conspecific playback in the spotted antbird. Condor, 104: 387-394 do not discriminate behaviourally between the calls of neighbours and strangers, and female collared lizards show no difference in their behaviour to neighbouring or unfamiliar females.Husak, J.F. and Fox, S.F., (2003). Spatial organisation and the dear enemy phenomenon in adult female collared lizards., Journal of Herpetology, 37, 211-215 http://www.jstor.org/discover/10.2307/1565856?uid=3739256&uid=2&uid=4&sid=21102326133101 Guinea baboon (Papio papio) males which live in gangs do not differ in their response behaviour toward neighbouring and stranger males and largely ignore any non-gang member, irrespective of familiarity; that is, they neither show a “dear enemy” nor “nasty neighbour” effect.Maciej, P., Patzelt, A., Ndao, I., Hammerschmidt, K. and Julia Fischer, J., (2013). Social monitoring in a multilevel society: a playback study with male Guinea baboons. Behav. Ecol. Sociobiol., 67(1): 61–68. DOI: 10.1007/s00265-012-1425-1 See also *Behavioral ecology *Dear enemy recognition *Territorality References Category:Behavioral ecology Category:Ethology Category:Evolutionary psychology Category:Sociobiology Category:Territorality